The present invention relates to photolithography.
In a photolithographic exposure process, a wafer surface is coated with photoresist, and the photoresist is illuminated by light passing through an optical mask having a pattern of clear and opaque features. These features define an illumination pattern of light and dark areas on the photoresist surface. At the light areas, the photoresist becomes more soluble (if positive) or less soluble (if negative) in a developer solution. The wafer is then placed into the developer solution to dissolve the soluble portions of the photoresist. The remaining photoresist provides a photoresist mask used in subsequent processing (e.g. etching, deposition, ion implantation, etc.).
In non-contact exposure, the optical mask is held at a distance from the photoresist surface to avoid mask damage. Due to bending of light rays, the pattern of light and dark areas in the photoresist does not always correspond to the pattern of clear and opaque features of the optical mask. The areas corresponding to the opaque features can thus be undesirably illuminated, reducing the contrast between the dark and light areas and/or reducing the dark areas' sizes.